This invention relates to motor controllers for induction motors and, more particularly, such motor controllers which have means for selectively adjusting the overload current value at which protective action is taken by the controller.
Different motors, of course, have different overload characteristics. Accordingly, a need exists to adjust the controller, so that it will respond to different overload currents.
In known circuits, the line current to the motor is monitored by means of one or more current transformers coupled to the active portion of the controller. The active part of the controller, however, requires voltage signals for control operations. Consequently, the input current monitoring signal must be converted to a proportionate voltage signal. In addition, the proportionate voltage signal must be adjustable to scale it for motors of different overload characteristics.
In known controllers, the above two functions have been done by means of separate circuits: one circuit for converting the input current to a corresponding voltage signal and another circuit for selectively altering the proportionate ratio of the input current monitoring signal and voltage control signal.